There has been an increased interest in alternative energy recently due to the draining oil reserves and environmental problems. The most spotlighted type of alternative energy is the solar energy power generation system.
Methods of producing electricity using solar energy include converting sunlight directly to electric energy and driving a generator with a heat engine using solar heat.
A solar power generator, which converts the solar energy directly to electricity, uses solar cells and is widely used because it is durable and easy to make the power generation system semi-automatic or automatic.
However, only some portions of sunlight, i.e., visible rays and a portion of infrared rays, can be used for the solar cells, and most photons in long wavelength infrared rays have low energy and thus cannot be used for generation of electricity but increase the temperature of the solar cells, thereby lowering the efficiency of power generation. Moreover, the solar cells are expensive and have a significantly lower efficiency of power generation than the heat engines.
Meanwhile, there have been active studies for a concentrating solar power (CSP) generator. External combustion engines used for the CSP generator include the Stirling engine and the turbine engine, which follows the Brayton cycle, and are generally higher in efficiency than the solar cells. Particularly, the solar dish type CSP generator using the Stirling engine has the highest efficiency of power generation.
When the Stirling engine is used, the heater of the Stirling engine is heated by the solar heat. The Stirling engine is a kind of external combustion engine that seals up operating gas, such as hydrogen or helium, in a space made with a cylinder and a piston and heats up and cools down this operating gas from the outside to reciprocate the piston to obtain mechanical energy. The Stirling engine has a high thermal efficiency with a cycle that is similar to the Carnot engine, which is an ideal engine.
However, since such a Stirling engine transfers the heat to the operating gas through the heater walls, the temperature that the operating gas can reach is limited, and energy is lost due to thermal conduction of a cooler. To minimize this problem, a high level of technology is required using hastelloy, which is a special alloy, but is not widely commercialized.
In order for an external combustion engine, such as the Stirling engine, to have a high thermal efficiency, the operating gas needs to repeatedly reach a high temperature and a low temperature. However, since the external combustion engine operated by the solar heat is constantly heated from the outside, unlike the internal combustion engine, in which a high temperature is reached instantly by combustion, the operating gas becomes expanded before it is sufficiently heated and reaches a high temperature, thereby lowering the efficiency of the heat engine. Particularly, in a linear heat engine that is operated by a free piston, strokes are restricted by heating compression pressure and cooling expansion of the operating gas, which acts on either side of the free piston.
Moreover, most heat engines convert a rectilinear motion to a rotary motion using a crank, according to thermal expansion of the gas inside the cylinder. During this conversion, much power is lost due to friction. Accordingly, there has been efforts to develop a linear power generator that is constituted with the free piston, but such efforts have been inhibited by the vibration and control problems.
In a power generator that converts mechanical energy to electrical energy after the solar energy is converted to the mechanical energy, the vibrations and torque generated when the solar energy is converted to the mechanical energy are transferred to a device that concentrates the solar energy, and thus the concentrated solar energy becomes out of focus. Moreover, these vibrations and torque overexert the power generator and shorten the life of the power generator.